Competitive pressures have increased the reliability and durability requirements for heavy duty diesel engines. In addition, performance and exhaust emission improvements have increased the thermal and mechanical loading on critical heavy duty diesel engine components.
To meet such requirements, the use of articulated pistons has grown in recent years. Conventional articulated pistons generally comprise a piston head and a separate piston skirt connected to both the piston head and a small end of a connecting rod by a common wrist pin. The piston head includes a combustion chamber formed in an end face and a hollow cavity for cooling the head. The hollow cavity is positioned between the combustion chamber and a peripheral pending leg which receives piston rings on its outer surface. Also, an annular rib extends from the inner surface of the head opposite the pending leg to surround the hollow cavity. The annular rib also joins the head to a pair of pin bosses with aligned holes for receiving the wrist pin.
For the purpose of improving the cooling of the piston head, a tray or trough is provided at the upper portion of the piston skirt. The tray opens towards the hollow cavity so as to partially close this cavity, thus forming what is known as a cooling gallery. When the engine is running, a cooling liquid such as lubricating oil is injected by a nozzle against the cooling gallery, through an oil inlet provided axially along the tray, thus partially removing heat from that region. The oil, impinging against the hollow cavity, flows down for collection by the tray. Due to the reciprocating motion of the piston, oil collected from the cavity is thrown against and around the cooling gallery thereby increasing the removal of heat from that region.
Piston assemblies as described hereinabove are disclosed in many prior letters patents, such as U.S. Pat. Nos. 5,144,884 issued to Kelly and 5,115,726 issued to Daxer et al., and U.S. patent application Ser. No. 689,244, filed Aug. 7, 1996.
Such conventional assemblies have shown good performance even when operating under severe conditions, such as very high temperatures and pressures and/or very high speeds, as often experienced in modern diesel engines. Although meeting the majority of current specifications, trends in this field have pointed out the need to reduce the weight and cost of such pistons to increase the power made available by the engine and to place the engine at a more competitive level in the market. In addition, many future engines will have increased specific power and pressures applied to the pistons.
U.S. Pat. Nos. 3,915,141 to Ottl et al. and 5,279,268 issued to Brink et al. both disclose articulated piston assemblies including a piston head connected to a skirt by one or more threaded fasteners. However, the use of fasteners increases manufacturing and assembly costs. Moreover, fasteners may not provide a sufficiently durable connection under high speed conditions.
U.S. Pat. Nos. 5,394,788 to Mendes et al. and 5,207,147 to Leites et al. both disclose articulated pistons including a piston head formed from two separate forged pieces which are welded together by, for example, friction welding. The piston head includes an annular rib forming one wall of an annular cooling cavity. However, the annular rib is formed with an excessively large thickness resulting in an undesirably heavy piston.
Consequently, there is a need for a lightweight piston head for an articulated piston assembly capable of handled high loads without undue distortion and a method for manufacturing the piston head which minimizes the costs and assembly time.